1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and a method for adjusting a capacitance value of a phase compensating capacitor.
2. Related Art
Many of circuits using operational amplifiers are ones with feedback (negative feedback). In a circuit with negative feedback, the negative feedback may change to positive feedback due to phase rotation in a high-frequency band to cause oscillation. For this reason, whether the circuit with a feedback loop has an element satisfying an oscillation condition (the stability of the circuit) is determined.
As a method for determining the stability of a feedback loop, there is available one using phase margin (see, e.g., Japanese Patent Laid-Open No. 2006-10596). In this method, a closed-loop circuit is opened at a point to form an open-loop circuit. A signal is inputted from one end of the open-loop circuit while varying frequency, an output from the other end is obtained, and a loop transfer function is calculated.
A unity gain frequency “fu” where gain is 0 dB (1×) is calculated from the gain-frequency characteristic of the loop transfer function, and the difference between a phase at the unity gain frequency “fu”, and a phase lag of 180° is calculated from the phase-frequency characteristic. The difference is phase margin.
If the phase margin is not less than a reference value (e.g., 15°), it is determined that the circuit is oscillation-free and stable. However, a circuit for phase characteristic evaluation is complicated and causes increase in cost.
The process is also performed of setting the capacitance value of a phase compensation capacitor of an operational amplifier to be large, making the phase margin large, and ensuring stability. However, along with increase in the capacitance of a phase compensation capacitor, the gain of a circuit decreases, and the bandwidth becomes narrow. If the capacitance of a phase compensation capacitor is large, slew rate decreases, and distortion occurs in an output waveform.
If device elements, each constituting a part of a circuit, are formed in the manufacturing process to vary widely, phase compensation capacitor samples need to be optimized (adjusted) individually.